Product sizing control in a grinding circuit closed by a separating means



Aug. 20, 1968 H. P. wHALx-:Y ETAL 3,397,844

PRODUCT SIZING CONTROL IN A GRINDING CIRCUIT CLOSED BY A SEPARATNG MEANSFiled Sept. 19, 1962 2 Sheets-Sheet l M R s R g E RQ,

INVENTORS M6. Jy mofm BY WJ-WL@ M #1.11 V ATToRNEY ug- 20, 1968 H. P.WHALEY ETAL 3,397,844

' PRODUCT SIZING CONTROL IN A GRINDING CRCUIT CLOSED BY A SEPARATINGMEANS Filed Sept. 19, 1962 2 Sheets-Sheet 2 /oa a 5 4 am CUM/' m @Nam qJ l/-mi j' a BY :i X;-

"y 'Y @Mnwrg United States Patent Olce 3,397,844 Patented Aug. 20, 19683,397,844 PRODUCT SIZING CONTROL 1N A GRINDING CIRCUIT CLOSED BY ASEPARATING MEANS Henry P. Whaley, Aurora, and Mark L. Hovland, HoytLakes, Minn., assignors, by mesne assignments, to Erle DevelopmentCompany, Cleveland, Ohio, a corporation of Delaware Filed Sept. 19,1962, Ser. No. 224,776

7 Claims. (Cl. 241-21) This invention relates to the art of subdividingsolid subdividable materials, more particularly, ores, and is concernedwith improved control for mill lines performing subdividing operationsthese latter including crushing (in one or more stages) followed bygrinding (in one or more stages). The invention is particularlyapplicable to so-called automatic grinding procedures. While notrestricted thereto the invention will, in the following, be describedwith special reference to processing of relatively low grade iron ore ofthe magnetic taconite type, found in relatively large deposits on theEastern end of the Mesabi Range, in Northern Minnesota, and elsewherearound the world, which illustrative processing includes mining crudetaconite, crushing in a plurality of stages, grinding in a plurality ofstages -along with a plurality of stages of magnetic concentration, and,nally, agglomerating into pellets and heat-hardening the pellets soformed.

Illustratively, the processing in the mill line begins as the crushedore (at, e.g., minus 1" in size) is drawn from a 2,000 ton tine ore binby vibrating feeders and subsequently conveyed to a 10' x 14 rod mill.The rod mill, driven by an 800 H.P. synchronous motor, grinds the ore,in an aqueous medium, to a mean size of 28 mesh, and discharges it ontoa vibrating screen which scalps out the plus 1A" material. The screenundersize is split to tWo 36 x 48" double drum electro-magneticseparators which reject a siliceous, essentially non-magnetic tailingand produce a rougher concentrate. This concentrate, in turn, isdirected, in aqueous slurry form, to a 10. X 14 ball mill driven by a1250 H.P. synchronous motor. The discharge of the ball mill is treatedby four 36" x 60" single drum permanent magnet separators which reject asecond tailing and produce a cleaner concentrate. This concentrate iscollected, as an aqueous slurry, in a sump and pumped by a 10" X S"rubber lined pump to a bank of five 14" wet classifying cyclones of Eriedesign, e.g., cyclones described in Herkenhoff Patent No. 2,756,878,issued July 31, 1956. The underflow from the cyclones is united with therougher concentrate as feed to the ball mill while the cyclone overflowis split to four 30 X 60" triple drum permanent magnet separators thatreject the third and final tailing and produce the final concentrate.This final concentrate is then sent on, in the form of a pumpableslurry, 4to the agglomerating plant to be made into high grade iron orepellets.

The purpose of the milling process just described is to liberate andrecover the magnetite contained in the crude magnetic taconite as a highgrade concentrate, having a constant percentage of minus 325 meshmaterial, at a maximum tonnage rate. We usually grind the ore to recovera concentrate assaying 89 to 90% minus 325 mesh.

It is extremely important that the sizing analysis of the concentrate be-held constant because of its effect on the subsequent filtering ballingand pelletizing phases of the agglomeration process carn'ed out in theagglomerating plant.

Since the final stage of grinding takes place in the ball mill, theoperating conditions of the cyclones close-circuiting the ball mill mustbe controlled to obtain the desired product sizing from a grinding line.It heretofore had been proposed to use the conventional method ofcontrolling cyclone overflow density lthrough manual adjustment ofdilution Water to the cyclone feed sump and manual adjustment of thetonnage set point on the automatic feed controller. The cyclone feedpressure was allowed to vary Within limits in order to maintain the feedsump at a constant level. This was accomplished automatically throughthe use of an air-bubbler type, level, sensing tube inserted in thesnmp. The back pressure created in the tube at different levels wasrelayed through pneumatic control to position the speed control arm of ailuid drive which varied the speed of the cyclone pump.

When We began investigating the possibilities of automatic grinding, ourearlier investigations were concerned primarily with testing different-types of devices for measuring the cyclone overllow density, one ofwhich was a radioactive gamma gauge. It was found that control of thecyclone overflow density alone could not give the consistency of productsizing results desired.

We have discovered that both cyclone feed density and the ball millcirculating load are affected by a change in any one (or more) of thefollowing variables:

(l) percent magnetic iron contained in the ore;

(2) hardness, or grindability of the ore;

(3) primary liberation size of the ore;

(4) `specific gravities of minerals making up the ore; and (5) screenanalysis of the rod mill feed.

In most grinding systems, the primary purpose is to grind the ore to apoint where it contains a specific weight percentage of a certain sizefraction. This point may be determined by economics, liberation sizecharacteristics of the ore, size requirements to perform furtherprocessing or any combination of the three. In the past, the commonpractice of obtaining a specific product sizing from a grinding circuitclose-circuited by cyclones was to measure the density of the cycloneoverflow, correlate this with sizing analysis of product samples, andthen subsequently adjust the density, if necessary, to hold the productsizing level desired. The basic -theory behind our control systemdiffers from any other control method in that our process is predicatedon Athe fact that the measurement and control of the cyclone feeddensity is by far the most important factor in maintaining constantproduct sizing from any grinding system close-circuited by cyclones.

We have discovered that so long as the cyclone feed density is heldconstant, the circulating load in the ball mill may be changed withinrelatively Wide limits without any noticeable effect on the finalproduct sizing.

It is an important Object of the present invention to provide anautomatic or essentially automatic grinding line control system formaintaining substantially full load on the grinding equipment whilstsimultaneously maintaining constant or substantially constant the pulpdensity of the cyclone feed.

The invention will now be described in greater particularity, withreference to the accompanying drawing, wherein FIG. l is a ow diagram ofa grinding line showing application thereto of the automatic grindingcontrol system of the present invention; and

FIG. 2 is a chart of curves showing the effect of changes in thecirculating load in the ball mill circuit on the sizing analysis of thecyclone feed and underow and the tonnage rate and density of the cycloneoverflow.

In FIG. 1 of the drawing, the pieces of equipment to which referencenumerals are applied are as follows:

1-2,000 ton fine ore bin 2-12 18 x 84" electro-vibrating feeders 3-24rod mill feed conveyor 3 4--10' X 14 rod mill (800 H.P.) 5 4' X 8vibrating screen 6-2 36" X 48 double drum electro-magnetic separators7-10' X 14 ball mill (1250 H.P.) 8-4 36" X 60" permanent magneticseparators 9-10" X 8 cyclone feed pump 9-8" cyclone feed pipelll-Demagnetized coil 11-5 14" Erie cyclones 12-4 30" X 60 permanentmagnetic separators And the illustrated automatic control systemcomprises the following components:

a-Gamma ray density gauge b-Density-gauge amplifier c-Densityrecorder-controller d-Timer for density circuit e-Tonnagerecorder-controller f-Timer for tonnage control circuit g-Motor drivenrheostat (feeder amp. control) h-Belt scale (tonnage sensing load cell)i--Level sensing bubbler tube i-Level indicator-controller k-Automaticair-controlled water valve The essential components of the automaticcontrol circuit are diagrammatically shown in the accompanying drawing.The heart of the control system is a gamma ray density gauge a,consisting essentially of a 150 millicurie source and receiver, mountedon the 8" cyclone feed pipe 9'. The source emits gamma rays through theiron, or rubber pipe and slurry to the receiver, mounted on the oppositeside of the cyclone feed pipe, where the emission is converted into anelectrical signal. The amount of signal created is inverselyproportional to the density of slurry within the pipe. This signal isdirected through a coaxial cable to an amplifier b where it is steppedup and sent on to a density recorder-controller c. Here it is convertedinto terms of specific gravity units and recorded by a pen on a 24-hourcircular chart graduated from 1.50 to 1.90 SGU (specific gravity units).

The control portion of the unit consists of four mercury switches thatare fixed relative to a density set point indicator to give ve densitycontrol zones; +0.02 and lower (-3% zone), 0.02 to 0.01 P/2% zone), 0.01to +0.01 (neutral zone), +0.01 to +0.02 (=11/2% zone), and +0.02 andhigher +3% zone). From this controller, an electrical signal is sentthrough a timer to a set point positioner-relay cabinet, that allows amotor driven set point indicator in the automatic tonnagerecorder-controller to be positioned up or down 11/2 or 3% dependingupon which percentage control zone the density pen is recording in. Thetimer in this circuit is set so as to allow a pre-determined amount oftime to elapse after each tonnage change before another change can bemade. This time lapse represents the process time lag inherent inbalancing out the ball mill circuit and is tentatively set at 40minutes.

The tonnage fed to the grinding line is controlled by the automatictonnage control system at the set point value as positioned by signalsfrom the density control system. This tonnage system consists of a beltscale mounted on the rod mill feed conveyor, atonnage-recorder-controller, a tonnage system timer, and a motordriven-rheostat that controls the amperage and subsequently the outputof the vibrating feeders.

Also incorporated with this automatic grind control system is a novelmethod of controlling the sump level. The speed of the cyclone feedpumpis now held constant while the bubbler tube is switched over tooperate an air-actuated water valve. The controlled discharge rate ofcyclone sump make-up water from this valve now maintains the sump at aconstant level.

We have found that when operating -by automatic control, the screenanalysis of the total mill concentrates can be held Within very closelimits. Table I, following,

TABLE I.-CQNCENTRA1OR IRODUCTION One hour mill concentrate grind results(percent-325 mesh) Date shows typical sizings obtained from millconcentrate samples taken at one-hour intervals over a four day period.It will be noted that during this period, out of 96 one-hour samples,the coarsest assayed 87.96%, while the finest assayed 90.50%, -325 mesh.After a month of operation, it was concluded that the `automaticgrinding system can indefinitely hold the individual one-hour sizinganalysis to within plus or minus l1/2% of the desired value. Based onthis, mill concentrate sampling need not be done oftener than at fourhour (or, even, eight hour) intervals.

The density set point values may require some adjustment from time totime. On the average, these adjustments amount to a correction of about0.005 density units once every four to ve days.

The automatic grinding system has resulted in the realization of thefollowing benefits:

(l) The closer control of mill concentrate sizing analysis makes itpossible to maintain a very exact control of the subsequent filteringoperation, resulting in close control of green ball moisture and,subsequently, a higher quality pellet.

(2) The automatic system reduces the operating manpower required in thegrinding and concentrating process, with an accompanying reduction inoperating costs.

(3) The increased reliability of predicting product sizing analysisallows for a reduction in sampling, sample preparation, and analyticaldetermination of mill concentrates.

(4) The constant product sizing enables the mining operators tocorrelate'their grading more accurately with mill results, and therebynarrow the fluctuations in chemical analysis of the concentrate.

(5) Automatic operation maintains maximum tonnage throughput at alltimes.

In regard to the specific features of the above described controlcircuit, the following statements may be made:

(l) The automatic system is based on the measured control of cyclonefeed density, and is capable of sensing and correcting for any change inore characteristics that may occur.

(2) The gamma ray density gauge provides an accurate on-line measurementof the critical cyclone feed density without coming in contact with theslurry and is thereby able to operate trouble-free insofar as pluggingand wearing out are concerned.

(3) The maintenance of the electrical control system is extremely low.

(4) The tonnage set point changer provides optimum flexibility inchanging the feed rate to compensate for ore characteristics changes.

FIG. 2 is composed of data curves which emphasize the importance thatconstant measurement and control of cyclone feed density plays inproducing products of constant sizing analysis from a closed circuitgrinding system. The curves are made up from daily results of samplestaken and data collected over a five-day sampling period. During thisperiod the circulating load (i.e., ball mill tonnage) was changedpurposely cach day in definite steps by varying the number of cyclonesoperating at 20 p.s.i. pressure. In this experiment, the cyclones feeddensity was held constant at .74 SGU during the entire period over whichthe ball unit tonnage was caused to change. In other words, the firstday test was conducted with 5 cyclones thereby furnishing a ball milltonnage of about 510 t.p.h.; the second day, 4 cyclones with a ball milltonnage of about 415 t.p.h.; and so on, down to one cyclone operation.It is significant that with these variations in ball mill tonnage, thecyclone overiiow product remained constant in size analysis at 85% minus325 mesh and that the final concentrate analysis remained constant at86% minus 325 mesh, due to the -fact that the cyclone feed density washeld constant.

The top or capacity curve reveals how the new feed tonnage had to bereduced with each step in order to hold the feed density of the cycloneat a constant value. The remainder of the curves located at the bottomhalf of the chart show what happens to the sizing analysis of thecyclone feed and underow and the tonnage rate and density of the cycloneoverliow as the circulating load changes in the ball mill circuit. Theimportant fact here is that these analyses, tonnage rates, anddensities, vary considerably as the circulating load changes but that byholding a constant cyclone feed density the sizing analysis of thefinished product from the circuit continues to be constant.

This chart, then, further reveals that, in reality, control ofcirculating load or control of cyclone overflow density are not theparticular rates or measures to be regulated in order to obtain optimumcontrol of a grinding circuit and resultant optimum product sizinganalysis, but rather that control or cyclone feed density is the truepulse of the circuit.

We claim:

1 In subdividing and concentrating an ore in a grinding circuitincluding a ball mill, which grinding circuit is close-circuited by acyclone separator, the improved method of ensuring constant productsizing from the circuit which consists in substantially constantlymeasuring the density of the feed to the cyclone separator andcontrolling the circuit at a predeterrnned density value by varying thenew tonnage rate to the grinding circuit in specified increments atspecified time intervals the variation being inversely proportional tovariation in the density of the feed to the cyclone separator.

2. The improved method defined in claim 1, characterized in thatmeasurement of the density of the feed to the cyclone separator iseffected by constantly directing a substantially constant volume ofemitted gamma radiation in a direction generally transverse to a owingstream of aqueous slurry being fed to said separator, substantiallyconstantly converting into an electrical signal the amount of emissionwhich passes through the stream, and substantially constantlydetermining density of feed in terms of the intensity of said electricalsignal.

3. The improved method defined in claim 1, further characterized in thatany ascertained change in the density of the feed to the cycloneseparator is compensated by an inverse change in the tonnage rate of theore feed to the ball mill.

4. In a closed-circuit ore pulp grinding and concentrating apparatusincluding, in series communication with each other, a ball mill, amagnetic separator, a sump, a cyclone separator, feed pump meansincluding a cyclone feed line for lifting pulp from said sump to theinlet of said cyclone separator, an underflow feed conduit from saidcyclone separator tol said ball mill, and variable means for deliveringraw ore feed to said ball mill, the improvement which consists in theprovision of a gamma ray density measuring device, providing a signalinversely proportional in intensity to the density, in association withsaid cyclone feed line, and of means responsive to said signal, forcorrespondingly varying the rate of delivery of raw ore feed by saiddelivery means.

5. In subdividing and concentrating a solid material in a grindingcircuit, which grinding circuit is close-circuited by a separatingdevice, the improved method of ensuring constant product sizing from thecircuit which consists in substantially constantly measuring the densityof the feed to the separating device and controlling the circuit at apredetermined density value by varying the new tonnage rate to thegrinding circuit in specified increments at specified time intervals thevariation being inversely proportional to variation in the density ofthefeed to the separating device.

6. The improved method dened in claim 5, characterized in thatmeasurement of the density of the feed to the separating device iseffected by constantly directing a substantially constant volume ofemitted gamma radiation in a direction generally transverse to a iiowingstream of aqueous slurry being fed to said separator, substantiallyconstantly converting into an electrical signal the amount of emissionwhich passes through the stream, and substantially constantlydetermining density of feed in terms of the intensity of said electricalsignal.

7. In a closed-circuit grinding and concentrating apparatus having a rawfeed delivery means, the improvement which consists of a gamma raydensity measuring device providing a signal inversely proportional inintensity to the density of a slurry produced by grinding; and meansresponsive to said signal for correspondingly varying the rate ofdelivery of raw feed by said delivery means.

References Cited UNITED STATES PATENTS 2,965,316 12/ 1960 Henderson etal 241-34 3,352,499 11/ 1967 Campbell 241--21 3,094,289 6/ 1963Fahlstrom et al 241-34 2,308,917 1/ 1943 Hardinge 241-34 X 2,534,65612/1950 Bond 241-34 X 2,499,347 3/ 1950 Adams 241-34 2,954,811 10/1960Hensgen et al 146-113 X 2,990,124 6/ 1961 Cavanaugh et al. 241-243,011,726 12/1961 Herz 241-33 X 3,022,956 2/ 1962 Haseman 241-243,114,510 12/1963 McCarthy et al. 241-34 OTHER REFERENCES Publication:Engineering and Mining Journal, vol. 162, No. 1, pp. 74-77, January1961.

GERALD A. DOST, Pr'maly Examiner.

1. IN SUBDIVIDING AND CONCENTRATING AN ORE IN A GRINDING CIRCUITINCLUDING A BALL MILL, WHICH GRINDING CIRCUIT IS CLOSE-CIRCUITED BY ACYCLONE SEPARATOR, THE IMPROVED METHOD OF ENSURING CONSTANT PRODUCTSIZING FROM THE CIRCUIT WHICH CONSISTS IN SUBSTANTIALLY CONSTANTLYMEASURING THE DENSITY OF THE FEED TO THE CYCLONE SEPARATOR ANDCONTROLLING THE CIRCUIT AT A PREDETERMINED DENSITY VALUE BY VARYING THENEW TONNAGE RATE TO THE GRINDING CIRCUIT IN SPECIFIED INCREMENTS ATSPECIFIED TIME INTERVALS THE VARIATION BEING INVERSELY PROPORTIONAL TOVARIATION IN THE DENSITY OF THE FEED TO THE CYCLONE SEPARATOR.